Sorry, no. The true test of a performer is when the pressure's on. She's not the first performer to have to give a performance while having "a lot going on".
Fleetwood Mac produced what is arguably their best work (Rumours) while the band members were breaking up...with each other!
Weird Al's parents died in a horrific accident - he took a week off from tour, got his act together, and went back out there.
I saw one singer give a great performance, and when I went backstage to talk with him afterwards, he croaked out a hello. He was fighting the flu, but he downed some cough syrup, and used the very last of his voice to put on a good show.
Britney is a performer who has "performed" for approximately four hours in the year 2007. This performance wasn't thrust upon her, despite what you might think. It was part of a careful plan for her "comeback". This plan has no doubt been in the works for months. I'm pretty sure the plan wasn't her doing (although she no doubt would've gotten all the credit had it gone well), but she must have been aware of it all along the line, and must to agreed to it. She recorded the songs that were designed to put her back on top, and this was going to be the show that pushed her over. And from the looks of things, she blew it. I'm sorry she wasn't "feeling it" because "she has things going on", but she's a performer, damn it - go out there and perform.